Linear positioning systems find uses in a wide variety of devices. In some cases such as in computer associated magnetic disk drives for positioning the read/write heads the requirements are particularly severe as far as actuation time and positioning accuracy are concerned. In computer systems utilizing disk drives lost positioning time can impair the overall computer performance and, thus, must be kept to a minimum. Positioning accuracy and repeatability determines the number of recording tracks that can exist on a disk and, therefore, directly affects the data packing density. Accordingly, the linear positioning system in a disk drive must operate at high speed as well as high accuracy.
A common prior technique used in disk drives included the use of a rotary stepping motor in combination with a lead screw arrangement to form a linear stepper. Such linear steppers are usually operated open loop and provide satisfactory results when repeatability constraints are not severe. However, in the better grade disk drives repeatability and positioning accuracy on the order of 0.3.times.10.sup.-3 inches or better is required. Linear steppers operating open loop and made using conventional manufacturing techniques normally cannot meet these requirements.
Linear induction motor systems have been proposed operating in a closed loop positioning mode. However, such systems are not easily implemented because of the need for a slip angle between the stator moving field and the moving induction unit in order to generate force. An accurate system with slip angle control becomes costly and complex and, even then, is difficult to operate with precision.
Another common technique has relied upon a moving coil actuator in a closed loop control system. Such systems are capable of relatively fast response and good accuracy. However, they are costly and generally not suited for high volume production. In such systems the moving coil is relatively large in diameter and therefore suitable for large disk file systems but not for the smaller disk drives of the type generally used with flexible recording diskettes. Furthermore, the moving coil systems have inherently poor heat dissipation and, therefore, difficulties are experienced with a rapid succession of movements.
An object of the linear positioning system according to this invention is to provide a system having a relatively fast response and high degree of positioning accuracy and repeatability.
Another object is to provide a linear positioning system suitable for high volume production at a moderate cost.
Another object is to provide a linear positioning system having good heat dissipation so that a rapid succession of movements can readily be achieved.
Another object is to provide a linear positioning system having a low profile suitable for use in compact disk drives.